It is known in fuel and oil filters (De-OS 31 03 723, DE-OS 28 40 117 and U.S. Pat. No. 4,253,954) that intermediary spaces can be located between filter elements in order to remove substances from the intermediary space, e.g. water or a first filtrate, separated in a first filter stage.
The basic concept filter unit, filter element (base filter element, large filter element), filter housing and filter device are used in the test concerning the state of the art according to the following definitions:
The sum of the filter-active individual layers, including the protective filter layers (protection during machining and during the insertion into the device) and drainage layers, of an individual filter element are called FILTER UNIT in the following.
A filter unit equipped with safety member (e.g. backwash protection and core in filter cartridges) which unit was processed in such a manner (e.g. sealing of the front sides, positioning of adapters) that it can be inserted in a filtration-active manner into a filter housing either directly (as based filter element) or after joining together several base filter elements to large filter elements is called a FILTER ELEMENT in the following.
One or several filter elements are inserted in a FILTER HOUSING and operated connected in parallel.
In a filtration device, for example, several filter housings are connected in series. The filtration process is controlled with appropriate fittings, peripheral devices and media and energy sources.
The invention relates for example, to integrated, tubular filter elements for the filtration of very different types of fluids (liquids and/or gases with various components--from true solutions via colloidal solutions via suspensions, emulsions, aerosols, smokes, etc. to liquid or gas mixtures).
In known filter elements, the object of the filtration is the separation of all or certain components of the fluid. Either the permeate (filtrate) or the retenate (concentrate) or both are of interest, depending on the application.
The quality and quantity of the separation is directly related to the filter media and filtration methods used.
The filter media can exhibit an absorptive, absorptive, electrochemical, catalytic and mechanical (deep action, sieve action) action. Such filter media are currently used in many forms as powder, granulate, staple fiber or monofil fiber mesh, threads, plane filters (round filters, sheets, roller item), filter mats, filter layers and filtration bodies which are sintered, mailed or produced in some other manner.
A distinction is also made between deep filters and sieve filters. The former are characterized in that the exclusion quantity is not absolutely considered and physical parameters influence the separation quality in a decisive manner. The classic sieve filters comprise, in addition to defined fabrics (in contrast to non-woven), the so-called membrane filters with the separating quality of microfilters, ultrafilters and filters for reverse osmosis. The class of "dense" membrane filters comprises filters for the separation of gases, prevaporation, ion exchange membranes and the like.
The use of these filter media has rapidly increased in recent years. This is particularly true for the group of membrane filters, which can be manufactured and processed in such a stable manner now that filter elements with sufficient filter area can be produced and can be combined in a modular design to quite large-area systems. The latter are frequently used in combination with deep filter systems, whereby in most instances the deep filters are used in the prefiltration state and have the task of precleaning the fluid in such a manner that the safety stage or final filtration stage can be economically employed.
The service life (flowthrough amount until irreversible blockage) and therewith the economy of the entire filtration system an be optimized as regards usage by means of stepped and specific prefiltration and appropriate graduation in the final filtration. The area of application of the aforementioned filter elements is very broad. They are indispensible in the pharmaceutical, electrical and semicondutor industries, biotechnology, medical technology, chemical industry, energy-supply industry, environmental protection and in the food industry (diaries, beverages, primary food products).
Such tubular filter elements, also designated in the ar as filter cartridges, are known in very many designs, in pleated, wound form with a great variety of stepped, multi-layer filter materials. The following publications regarding the state of the art constitute examples for such filter cartridges:
DE-OS 1 950 068 PA1 U.S. 3,452,877 PA1 DE-AS 1 029 345 PA1 DE-OS 33 18 940 PA1 DE-OS 26 45 634 PA1 GB-OS 2,152,471 PA1 Service life PA1 Flowthrough per area and time PA1 Testability
In the specific prefiltration or stepped filtration, it is customary at the present time to connect different filtration stages in series in a discontinuous or continuous manner. Thus, for example, in the making of wine, a coarse or fine separation by centrifuge and a discontinuous clarification of the wine are frequently performed and the wine is subsequently filtered via a so-called layer filter (open system with layer filter plates in a layer filter press, or closed system with "plates" of layer filter units arranged like Chinese lanterns) before the so-called final or sterile filtration via membrane filter cartridges. The number of the necessary filter units can be coordinated with each other by means of differently dimensioning the series-connected housings and varying the housing equipment.
The disadvantage of such specific and stepped filtration is on the one hand the discontinuity and on the other hand the plurality of systems and methods which requires a plurality of devices, especially filter housings, to receive the various filter elements. Every additional device or housing increases not only the space requirement but also the investment expenses for devices, fittings and peripheral devices as well as the expenses for operation and maintenance (setting periods, time required for rinsing, moistening, testing, sterilizing, cleaning). The danger of secondary contamination and process disruptions and the associated product losses and/or product damage increases to a degree which is more than proportional.
A first attempt to render specific and stepped filtration more economical is constituted e.g. by filter cartridges in whose filter unit deep filters are arranged over one another in a laminar and multi-layer fashion in front of membrane filters and/or membrane filters with a differing separating characteristic (pore size, cut-off) are arranged in series within an element.
A disadvantage of this solution is, e.g. in the case of pleatings, the fact that the various filter-active layers of a filter unit always function in a ratio of 1:1. It is correspondingly difficult to find filter media which permit an optimum utilization of the filter capacity (all layers blocked in the same manner). Secondly, the number of maximally integrally cooperating layers is limited from a purely manufacturing viewpoint. As the number of filter layers which function in a filter unit increases, the risk also increases that during the testing of the filter efficiency of the filter element, valuable, good material must be discarded on account of a small defect in a single filter layer. Moreover, as the the number of filter-active components increases, it becomes more and more difficult to check the efficiency of the individual component in conjunction with the finished filter unit and to interpret the test results.
These problems can be explained using the exampled of pleated membrane filters.
Microfilter systems, especially for sterile filtration, are subjected before and after filtration to so-called integrity tests which are intended to assure filtration safety. These tests include the bubble-point or bubble-pressure test, pressure-resistance test and the diffusion test. These tests correlate with destructive charge test, the so-called bacteria-challenge tests in which the output capacity of membrane filters or membrane filter products is determined with standardized test germs according to a standardized test method.
In spite of these physical integrity tests being automated, this type of test involves considerable uncertainty. This uncertainty increases as the filter area is enlarged to a degree which is more than proportional.
Large (individually testable) filter areas are generated today as follows: Rather small base elements are individually tested and then combined axially to larger units (welded, adhered, sealingly inserted...). These large filter units are again subjected to a test. Filter units which test defective or non-functional are discarded.
A further enlargement of area (output) is performed by connecting in parallel several such large filter units (e.g. multiple cartridges) in large housings.
Aside from the great testing expense, this design has the great disadvantage that in spite of automatzation, only a collective measurement is performed, that is, it is just as difficult to distinguish between individual defects of individual filter elements in a large or multiple housing as it is between individual defects of individual base elements which make up large filter element. In the case of large areas, aside from the fact that economy is no longer present even at low separation rates, an interpretation of the test result is becoming more and more difficult and critical, at least for the pressure retention and diffusion test.
In these methods, the volume of the test gas diffusing through the filter unit of the filter element to be tested or the pressure drop which this causes is determined on the approach flow side. The allowed diffusion volume theoretically increases proportionally to the filter area. Practice as shown, however, that this diffusion volume is also considerably influenced by other parameters. These parameters include temperature, layer thickness, moistening behavior, the chemical nature of the filter media, the chemical nature of the media with which the filter unit is operated. Thus, the diffusion volume is composed of very different components. Whereas diffusion caused by genuine defects can be unambiguously recognized in small-area systems due to the magnitude, this is no longer possible in large-area systems since allowed diffusion and diffusion caused by small but damaging defects are of the same magnitude. This is also the reason why a visual determination of the bubble point is extremely difficult in large housings.
It can be determined that all suggestions for testing filter element in a large housing individually and economically have not been successful.
It is therefore frequently necessary, if the integrity test has failed and after operational disturbances during production (filtration), to test the operational capacity of the individual filter elements. To this end, the filter housings must usually be emptied, the filter elements taken out and individually tested. As the number of filter housings increases, the cost for time, personnel and expresses naturally increases, as does the amount of product loss and the danger of product damage and secondary contamination.
Another disadvantage of filter units of a known type connected in series invarious housings is the fact that in the case of failure or defect of only one series connected element, frequently the entire following filter housing is also involved, that is, all elements of this housing are prematurely blocked.
Large filter areas are becoming more and more indispensible today if filtration as separation method is to remain economical and competitive.
In addition to the separating quality, three criteria determine th economy and reliability of filtration systems:
The problem of testability has been often described.
The problem of service life has also been explained in detail. It remains to be noted here that the attempt is being made more and more often currently to regenerate "blocked" filter units or filter units which are no longer filter-active. This occurs e.g. by means of the use of chemical cleaning agents, which is naturally limited by the chemical compatibility of the system as well as by means of washing processes with active washing media in or counter to the direction of filtration.
The disadvantages of the washing methods reside in the fact that during washing, filter units or devices which are downstream in the direction of filtration can be excessively loaded if bypass lines are not installed, which require a complicated apparatus and are usually also expensive. On the other hand, a backwash (counter to the direction of filtration) involves the risk of destroying the filter element. This is especially critical at the end position and should particularly be avoided if the end position itself is not blocked but rather an upstream filter element is blocked which can not be individually controlled.
The problem of flowthrough per time and area is naturally also coupled to the separating characteristic, the testing problem and the service life problem. Usually, this problem is solved by means of a high, freely accessible area with appropriate dimensioning and shaping of inlets and outlets as well as of the flow guidance. In the case of gas filtration, this means enlarged inner core diameter (loss of area) in the case of commercially available filter cartridges and many individual units of low overall height (testability, apparatus cost!) on one housing plane.
The disadvantage of the solution of the problem is congruent with the above comments.
The invention therefore has the problem of eliminating the disadvantages of the known filter units or filter elements with simple means, that is, to create a compact integral filter for specifically stepped filtration (multiple filtration) which filter makes good use of the given filtration space in a surrounding housing, makes optimum use of the filter materials used, permits directed solutions to problems, is economical to manufacture and is easy to service without the solution of the aforementioned partial problems worsening the integral in relation to current individual filter elements as regards its possible testability according to varied testing criteria and without the possibilities of supplying and emptying the integral filter being worsened.